This invention relates generally to the field of control of antilock braking for vehicles, and in particular, to a system and method for ABS stability control.
Antilock braking systems (ABS) are becoming more common on vehicles, particularly automobiles and light trucks. When the driver applies the brakes, ABS monitors whether a tire is skidding or about to skid, and automatically releases the brakes for a short period, so the wheel is allowed to rotate, rather than skid. The ABS control system controls braking pressure in either an applied, hold, or release state, cycling from one to the other throughout the stop, depending on a wheel""s skid condition.
Typically, skidding is monitored at each of a vehicle""s four wheels by measuring wheel speed and looking for a sudden deceleration, indicating the wheel is no longer rolling, but is stopping and skidding. Braking pressure is typically controlled to both rear wheels as a single channel for stability, while braking pressure to each front wheel is controlled independently.
Although controlling the breaking pressure to the rear wheels as a single channel helps maintain vehicle stability, it has the disadvantage of reducing rear braking efficiency and increasing stopping distance because it ignores differences between the rear wheels. One rear wheel may have braking pressure applied or released because of the state of the other rear wheel, controlled on the same channel. One rear wheel may be more worn or be adjusted differently than the other. One rear wheel may be traveling further than the other in a turn. Thus, under single channel breaking control the ABS control state of one rear wheel may be inappropriate for the conditions sensed at that wheel.
Controlling the braking pressure to the front wheels independently greatly reduces the stopping distance, but may have the disadvantage of increasing vehicle yaw. One front wheel may be in the apply mode at the same time the other front wheel is in the release mode, producing a yaw moment on the vehicle. Under certain conditions for certain vehicles, the yaw may induce steering oscillations, making the vehicle more difficult to control and distracting the driver during a stressful situation. The effect may be most pronounced at high vehicle speeds on dry surfaces.
Accordingly, it would be desirable to have a system and method of ABS stability control that overcomes the disadvantages described.
One aspect of the invention provides a method for ABS stability control, comprising the steps of determining whether a first wheel is in an ABS mode; determining whether the first wheel is in an apply mode; determining whether a second parallel wheel is in the release mode; calculating an adjusted wheel slip if the first wheel is in the ABS mode, the first wheel is in the apply mode, and the second parallel wheel is in the release mode; and determining a control mode for the first wheel using the adjusted wheel slip.
Another aspect of the invention provides a method for ABS stability control using the rear wheels, comprising the steps of determining whether a first rear wheel is in an ABS mode; determining whether the first rear wheel is in an apply mode; determining whether a second rear wheel is in the release mode; calculating an adjusted wheel slip if the first rear wheel is in the ABS mode, the first rear wheel is in the apply mode, and the second rear wheel is in the release mode; and determining a control mode for the first rear wheel using the adjusted wheel slip. The step of calculating an adjusted wheel slip further comprises the steps of selecting a slip offset as the lesser of a first constant K1, or a second constant K2 times vehicle speed with the product added to a third constant K3; and adding the slip offset to the measured wheel slip from the first rear wheel to calculate the adjusted wheel slip.
Another aspect of the invention provides a method for ABS stability control using the front wheels, comprising the steps of determining whether a first front wheel is in an ABS mode; determining whether the first front wheel is in an apply mode; determining whether a second front wheel is in a release mode; determining whether a vehicle speed is greater than a speed constant K4; determining whether a vehicle acceleration is less than an acceleration constant K5; calculating an adjusted wheel slip by adding a slip constant K6 to a measured wheel slip for the first front wheel if the first front wheel is in the ABS mode, the first front wheel is in the apply mode, the second front wheel is in the release mode, the vehicle speed is greater than the speed constant K4, and the vehicle acceleration is less than the acceleration constant K5; and determining a control mode for the first front wheel using the adjusted wheel slip.
The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.